Synthesis gas for production of for instance methanol, dimethyl ether (DME) or liquid hydrocarbons via for instance Fischer-Tropsch synthesis, may be produced from carbon-containing feedstock such as natural gas LPG, liquid hydrocarbons including heavy hydrocarbons, or solid feedstock such as coal. The carbon-containing feedstock is reacted with steam and/or air, enriched air, or oxygen at high temperature during steam reforming, autothermal reforming, catalytic partial oxidation or combinations thereof.
In the conventional steam reforming process natural gas or light hydrocarbons are reacted with steam in the presence of a catalyst based on nickel or noble metals. Temperatures at the reactor outlet of up to 950° C. are obtained.
During autothermal reforming (ATR) or catalytic partial oxidation (CPO), natural gas or other hydrocarbons are reacted with steam and an oxidant (air, enriched air, or oxygen) in the presence of a catalyst based on nickel or noble metals. Temperatures up to 1100° C. are usually obtained at the outlet of the reactor. During non-catalytic partial oxidation (POX) of natural gas, light hydrocarbons, heavy hydrocarbons or solid feedstock such as coal (also referred to as gasification) is reacted with an oxidant (air, enriched air or oxygen) and outlet temperatures from the reactor of up to 1400° C. are obtained.
These processes are well known to those experienced in the art. A comprehensive description of the individual processes and relevant variations and combinations thereof is given by e.g. Aasberg-Petersen et al. Fischer-Tropsch Technology, Stud. Surf. Sci. Catal. 152 (2004) 258-405, edited by Steynberg, A. P. and Dry, M. E.
In processes based on steam reforming and/or autothermal reforming or catalytic partial oxidation the composition of the synthesis gas may be an equilibrium mixture of hydrogen, carbon monoxide, carbon dioxide, methane and steam established at the outlet temperature and pressure of the last catalytic reactor according to the reactions:Steam reforming: CH4+H2O=CO+3H2  (1)Water Gas Shift: CO+H2O=CO2+H2  (2)
In partial oxidation the equilibrium may be established at a temperature somewhat lower than the outlet temperature from the reactor. Hydrocarbons other than CH4 will generally be present in synthesis gas produced by any of the methods only in small or insignificant amounts. However, certain other components may also be present in trace amounts as impurities with possible detrimental effects in downstream processes, especially if the feedstock or the oxidant contains nitrogen. Impurities of special interest are ammonia, hydrogen cyanide and formic acid.
These impurities will be present in amounts corresponding to establishment of equilibrium (at the same conditions as the equilibrium for reactions (1) and (2)) for the following reactions:3H2+N2=2NH3  (3)CO+NH3=HCN+H20  (4)CO+H2O=HCOOH  (5)
The concentration of ammonia may be up to a few hundred vol ppm, whereas the concentration of hydrogen cyanide and formic acid will normally be less than 100 vol. ppm.
After leaving the reactor where the synthesis gas is formed, the raw synthesis gas is cooled in one or more steps to a temperature where most of its content of water vapour condenses. The first cooling step can be used to produce steam followed by cooling in air and/or water cooling. The condensate is separated, and the synthesis gas is sent to the section for synthesis of the final product e.g. methanol, dimethyl ether or hydrocarbons. The condensate will comprise dissolved gases including carbon oxides, most of the ammonia, and almost all of the formic acid. The pH of the condensate will typically be around 7.
Hydrogen cyanide will at this pH not be dissociated in the water, and it will be distributed between gas and condensate. The synthesis gas will thus, in addition to the main components hydrogen, carbon monoxide, carbon dioxide and methane, also contain traces of ammonia and hydrogen cyanide. The condensate will contain the dissolved gases comprising hydrogen cyanide, most of the ammonia and formic acid. This is undersirable since formic acid and formates are contaminants which are corrosive in downstream condensing units and cause additional load on water purification units or prevent reuse of condensate.
The content of ammonia, hydrogen cyanide and formic acid in both the synthesis gas and the condensate may cause problems in downstream process steps. In the synthesis of methanol or DME, ammonia and hydrogen cyanide may be converted to methyl amines, which are undesired in the products and must be removed, e.g. by ion exchange. A more serious effect is seen in hydrocarbon synthesis by Fischer-Tropsch reaction, especially when catalysts based on Co are used, see e.g. U.S. Pat. No. 6,107,353. In such cases, ammonia and hydrogen cyanide may act as catalyst poisons by unfavourably affecting the performance of the synthesis catalyst.
Traces of ammonia are easily removed by washing with water. Hydrogen cyanide in the gas is difficult to remove by washing since the solubility in water is limited at the prevailing conditions.
The condensate is most often purified by flashing and/or stripping with steam followed by final purification by ion exchange. A survey of various concepts for stripping of process condensate may be found in J. Madsen: Ammonia Plant Saf. 31 (1991) 227-240. The presence of hydrogen cyanide and of formic acid in the synthesis gas and the process condensate is thus undesirable.
The removal of hydrogen cyanide from gases is described in the literature. It is thus known that Al2O3 is able to convert HCN in town gas plants, see for instance: Hydrolysis of HCN on different oxidic catalysts at 400° C., J. D. F March, W. B. S Newling, J. Rich, J. Appl. Chem 2, 1952, 681/4.
JP patent application no. 53-5065 to Nitto Chemical Industry K.K. discloses a two step process for treating a hydrogen cyanide-containing waste gas. The waste gases result from processes using hydrogen cyanide such as ammoxidation, electroplating, metallurgy industries and others. The first step comprises hydrolysing hydrogen cyanide to ammonia and carbon monoxide in the presence of a hydrolysis catalyst containing at least one element selected from the group consisting of aluminium, molybdenum, vanadium, iron, cobalt, nickel, copper, manganese, silver and lanthanum. The hydrolysing catalyst is preferably active alumina. Preferable is also alumina, MgO or TiO2 carrying alkali and/or alkaline earth metals. The second step is an oxidation step whereby ammonia and carbon monoxide are converted to nitrogen and carbon dioxide in the presence of an oxidation catalyst.
Examples are given in which the hydrolysis catalyst is alumina impregnated with either lanthanum nitrate, chloroplatinic acid or palladium chloride.
U.S. Pat. No. 6,107,353 discloses the removal of hydrogen cyanide in a catalysed hydrolysis step followed by a scrubbing step for removal of the ammonia formed. The hydrolysis catalyst comprises alumina, oxides of molybdenum and titanium in specific amounts.
The problems associated with the presence of formic acid in the condensate and the possible formation of formic acid in the catalytic reactor by reaction (5) are not discussed in the above disclosures and no solution is provided to this problem.
It is therefore an objective of the invention to provide a process by which hydrogen cyanide is removed from the wet synthesis gas before water vapour is condensed so that the content of hydrogen cyanide in both the dry synthesis gas and the condensate is reduced to a lower level.
Another objective of the invention is to provide a process which, in addition to the removal of hydrogen cyanide from the synthesis gas, also removes the formic acid and its derivatives formed in the synthesis gas generator by reaction (5).